Fluid actuated operators associated with valves are known in the art. At least some of the prior art actuators used spring and pistons where the maximum spring load occurred when the valve is in its closed position. Other prior art teachings for valve actuators use pistons but omit spring means urging the pistons in a given direction.
Still other valve operating mechanisms use toggle links to increase output force as compared to input force. Examples of these are described in U.S. Pat. Nos. 1,007,275; 1,603,005; 2,354,987 and 2,370,604 .
Another device incorporates a toggle link associated with a pneumatic type diaphragm to actuate valves, this also incorporates a manual override and is described in Bulletin No. 377E (copyrighted 1976) and entitled "The .SIGMA.F System of Control Valves" of Masoneilan International, Inc., Norwood, Mass. 02062 .